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Waste heat recovery system
Introduction to Waste Heat Recovery System In the process of treating VOCs in RTO (Regenerative Thermal Oxidation Furnace), a large amount of heat generated by high-temperature combustion (usually 800-850 ℃) can be recovered into four heat sources: hot air, hot water, steam, or thermal oil through heat exchange technology. The following provides a detailed analysis of these four recycling methods from both their principles and advantages:
1、 Reduce energy costs and achieve 'energy self-sufficiency' Reduce external energy consumption: The recovered waste heat can directly replace the heat generated by enterprises through external energy sources such as electricity, natural gas, and coal. For example, hot air is used for the drying line in the spraying workshop, which can replace electric heating or gas drying equipment. Calculated based on a medium-sized drying line (with a power of 50kW), it can save about 400000 kWh of electricity per year and save more than 300000 yuan in electricity bills; If steam recovery meets 50% of the steam demand of enterprises (such as the steam used in chemical reaction vessels), it can reduce the operating time of gas boilers and save natural gas costs by tens of thousands to millions of yuan per year (depending on the scale of the enterprise). Energy cascade utilization: The high-temperature heat from RTO is first preheated into the air through a thermal storage body (core energy-saving link), and the remaining heat is then used for heat generation (hot air/hot water/steam/thermal oil), achieving "primary energy, multi-stage utilization". The comprehensive energy utilization rate is increased from 90% of traditional RTO to over 95%, further reducing the energy consumption per unit product. 2、 Enhance environmental benefits and support green production Reducing carbon emissions: Waste heat recovery reduces the consumption of purchased energy, indirectly reducing carbon emissions from fossil fuel combustion or electricity production (especially thermal power). Taking a certain spraying enterprise as an example, if the annual heating demand (equivalent to 100 tons of standard coal) is met through waste heat recovery, it can reduce CO ₂ emissions by about 260 tons per year, helping the enterprise achieve its "dual carbon" goals. Compliant with environmental policies: Some regions have subsidies or tax incentives for "waste heat utilization" (such as subsidies for energy-saving technology renovation projects), and waste heat recovery can reduce the dependence of enterprises on traditional heat sources, reduce the emissions of pollutants such as SO ₂ and NOx generated by fuel combustion, and avoid the risk of environmental penalties. 1、 Hot air recovery (direct/indirect heat exchange) Principle Description Hot air recovery is the process of heating the high-temperature purified gas (or intermediate high-temperature gas) emitted by RTO through a heat exchange device to generate hot air at room temperature. It is divided into two modes: ·Direct heat exchange: If the purified gas is non corrosive and clean (such as single VOC components and no harmful residues after combustion), high-temperature gas can be directly mixed with the air to be heated through pipelines to quickly generate hot air (usually at a temperature of 100-300 ℃). ·Indirect heat transfer: Through metal tube or plate heat exchangers, high-temperature gas flows through the shell side, cold air flows through the tube side (or vice versa), and heat transfer is achieved through tube wall heat conduction to avoid gas mixing (suitable for purifying gas containing small amounts of pollutants). Advantages analysis ·High heat transfer efficiency: Air has a small specific heat capacity and a fast heating rate. The indirect heat transfer efficiency can reach 70% -85%, and the direct heat transfer is close to 100%. ·Flexible application: Hot air can be directly used for drying (such as drying lines in the spraying and printing industries), workshop heating, boiler combustion air preheating, etc., without the need for secondary conversion. ·The system is simple: only heat exchangers, fans, and pipelines are needed, with low investment costs (accounting for about 5% -10% of the total RTO investment), and easy maintenance (regularly cleaning the surface ash of the heat exchangers is sufficient). Practical Case
2、 Hot water recovery (indirect heat exchange) Principle Description Hot water recovery transfers the high-temperature heat from RTO to cold water (or low-temperature water) through a heat exchanger, raising the water temperature to 50-90 ℃ (standard for domestic or industrial hot water). Usually, shell and tube heat exchangers are used: high-temperature gas (such as RTO exhaust gas at 150-250 ℃) flows through the shell side, cold water flows in the opposite direction from the tube side, absorbs heat and heats up through the tube wall, and the outlet hot water can be stored in an insulated water tank for subsequent use. If the RTO temperature is higher (such as above 800 ℃ at the combustion chamber outlet), a finned tube heat exchanger (increasing the heat transfer area) can be used to improve the efficiency of hot water production. Advantages analysis ·High safety: Water as a heat exchange medium has strong chemical stability, no risk of combustion and explosion, and low-temperature hot water (<90 ℃) has low requirements for equipment materials (ordinary carbon steel is sufficient). ·Widely applicable: It can be used for hot water for enterprise employees' daily life, workshop floor cleaning, heating system replenishment and preheating, especially suitable for small and medium-sized enterprises without steam demand. ·Convenient energy storage: Short term storage of hot water can be achieved through insulated water tanks (insulation loss ≤ 5%/day), alleviating the time difference between RTO heat fluctuations and water demand. Practical Case
3、 Steam recovery (waste heat boiler/flash evaporation system) Principle Description Steam recovery requires the use of RTO's medium to high temperature heat (usually gas temperature ≥ 300 ℃) to heat water to boiling and generate steam (pressure generally 0.3-1.0MPa, corresponding to saturation temperature 133-184 ℃), with the core equipment being a waste heat boiler. High temperature gases (such as RTO combustion chamber outlet at 800-850 ℃) enter the boiler furnace and exchange heat with the boiler water through tube bundles. The boiler water absorbs heat and vaporizes to produce saturated steam, which is separated by a steam water separator and output. If the RTO heat fluctuates greatly, a flash evaporation tank can be used: high-temperature hot water (not boiling) is introduced into the low-pressure flash evaporation tank, and the pressure is reduced to quickly vaporize the hot water and produce low-pressure steam, improving the stability of steam production. Advantages analysis ·High energy density: Steam (especially high-pressure steam) has a high heat content (1 ton of saturated steam contains about 2700MJ of heat), which can be used to drive steam turbines for power generation, process heating (such as chemical reaction vessels), etc., achieving high-value utilization. ·Strong continuity: Through automatic control of boiler water level and linkage adjustment of RTO temperature, stable steam production can be achieved (pressure fluctuation ≤ ± 0.05MPa). 4、 Thermal oil recovery (indirect heat exchange) Principle Description Thermal oil (such as mineral oil or synthetic oil, resistant to high temperatures of 200-350 ℃) is used as a heat transfer medium to absorb the high-temperature heat of RTO through a coil heat exchanger (usually using the heat from the combustion chamber outlet or the middle section of the heat storage body at 700-800 ℃). After heating up, it enters the thermal oil circulation system to provide heat for equipment that requires high-temperature heating (such as reaction vessels and dryers). The cooled thermal oil is then returned to the heat exchanger for recycling. Key requirements: The heat transfer oil should have good thermal stability (to avoid high-temperature cracking), and the heat exchanger should be made of high-temperature resistant alloy material (such as 310S stainless steel). Advantages analysis ·Good high-temperature stability: Thermal oil can reach a working temperature of 200-350 ℃ under normal pressure, suitable for processes that require high-temperature heating (such as high-temperature reactions in the chemical and pharmaceutical industries), without the need to withstand high pressure like steam. ·Accurate temperature control: Temperature regulation of ± 1 ℃ can be achieved through the thermal oil circulation pump and temperature control valve, meeting the requirements of precision processes. ·Low heat loss: Thermal oil systems usually use insulated pipelines, with a heat dissipation loss of ≤ 3%, and can be transported over long distances (up to hundreds of meters), flexibly matching dispersed heat consuming equipment. Practical Case
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